Previous work reported by J. F. Massicott, et al, in Elect. Lett., 1990, vol. 26, pp. 1645-1646, has shown that optical gain can be obtained in erbium-doped silica fibers in the long wavelength range between 1570 and 1600 nanometers (L-band). It is more recently reported by Y. Sun, et al, in Proc. Opt. Amplifiers and Their Appl., PD2 Canada, July, 1997 and in Proc. ECOC., PDP 69, Edingburgh, U.K., September 1997, that by combining the gain in the conventional wavelength range between 1525 and 1565 nm (C-band) and the gain of the L-band, a bandwidth of 80 nm can be obtained using a split band EDFA structure.
The ultra wide band EDFA is physically divided into two (or more) sections. The first section includes a common amplification stage in order to achieve a good noise figure. In the second section the optical channels are split into separate parallel channels for amplification, one for the C-band and one for the L-band. Demultiplexing into the separate parallel channels may be accomplished by the combination of a circulator and a broad band fiber. A similar circulator and Bragg grating after the parallel channels can be used to re-multiplex the signals and an additional common output section to increase output power can be added, if desired.
While the common amplification stage improves noise figure, the improvement in the L-band is not as good as in the (-band because the gain of the first stage for the L-band is much lower than for the C-band. The loss associated with demultiplexing after the input gain stage increases the noise figure of the L-band more than the C-band. Accordingly, it would be advantageous to have a split-band optical amplifier architecture in which the input gain section yielded high gain for the L-band while maintaining high C-band gain.